Reason 1: If you look at ships sailing away from you, you can absolutely see that these ships submerge, this is because the Earth is round. Here is an example:http://www.smarterthanthat.com...
Pretend the brown object is a ship, and the bottom blue line is your viewpoint.
Reason 2: The North and South Hemispheres have different Constellations, for example the North has the Draco Constellation and the South has the Southern Cross, seen in the flags of Australia and New Zealand. If Earth was flat then we will see all of these constellations.
I have more reasons, I just don't want waste them right now on your lack of intelligence.

And by the way if you say if Earth was round that we will fall off, there is something called Gravity.

I thank my opponent for his cooperation in this debate, though I get the most overused and least understood proofs of a spherical earth. The age old "ships go over the curve" has been disproved several times over since the invention of the telescope, but for the sake of this debate, I'll show you some videos and provide a simple explanation.

I can understand why people long ago believed in a spherical earth. They saw boats appear to go over the horizon in all four directions. At a distance, the ship"s hull seems to disappear before the sails and those ancient geographers felt this would only happen if the ship was dipping behind the convex curvature of the spherical earth. This assumption is actually incorrect and if they would have timed or measured the distance that each boat had gone before dipping below the so-called curvature, they would have seen that the "ball" they thought they were on was far too small for their other calculations.

The reason for this is simply the result of perspective. This exact thing will happen on earth whenever something is moving away from the observer. Take a vehicle for example, on a long and flat stretch of a highway. If you watched a car that was driving away from you, the car would appear to slowly get lower and the tires would morph into the road. Soon the body of the vehicle would appear to be scraping the road. So, one may assume that the car has gone over some sort of curvature, but again, this is not correct.

One way to prove this is to bring out a pair of binoculars or even a telescope. When the object in question begins to morph or "sink" below the horizon, looking at it again with the aiding instrument will bring the item back into full view. Did the telescope manage to bend over the horizon to see the item in full again? Is it magically dragging the entire ship backwards over the horizon? Of course not. The binoculars were able to allow you to see further and therefore the item reappeared in full to your perspective.

So, the next time someone tells you that the ship going over the horizon is proof that we live on a globe" tell them about a little thing we here in reality must deal with called the Law of Perspective. The boat that they think has disappeared from sight can be seen again through a telescope and this was in no way caused by it going over a hump of curved water. Which as we know, is impossible anyway! The natural property of water is that it ALWAYS goes flat. ALWAYS. The railroad tracks going away from you don't get closer together, coming to a point, it only looks this way. It works on a vertical axis too. The telephone poles do not drop in height, they only seem to.

This, in combination with atmospheric refraction, which is very strong close to the ocean's surface, where waves constantly break, throwing salty water in the air. This bends the light even more, morphing any light that comes through. So that the ocean actually disappears at a certain point, and takes everything with it, blending it with the sky, this is where your mirage comes in. A globe earth proponent filmed this event, broke it up and put it online. this guy evaluates the entire video, and the results are very interesting.

Reason 2. Constellations This is probably one of the most ridiculous proofs I've heard yet. You think that just because we don't see the same stars as someone thousands of mile away from us, this somehow proves that we live on a ball. In the flat earth model, the stars are close. They are unthinkably huge balls of fire at unthinkably far distances. Because of this, they follow the same Visual laws as boats, the moon, the sun, everything. I feel the need now to provide a FAQ for anyone who hasn't done any research so that this debate doesn't get redundant for me, this is probably my 5th time explaining this. Note that all of this sites views are not mine, this is merely a reference point for baby steps. please further refute from there unless you have some new information. Also a video refutation to the top 10 ways popular videos everybody brings up in the google search, it's time to stop being lazy.https://wiki.tfes.org...;https://www.youtube.com...By the way, you do realize you are proudly waving the flat earth map on your flag? Lack of intelligence lol.Since burden of proof is shared, I would like to give 3 Empirical proofs of a flat earth.1. Flight pathsSome flight paths go in very unusual directions on the globe model, but make perfect sense on what is majorly accepted as the flat earth map (of course, the map can be wrong) I use Delta airlines, as this is the most reputable, and longer lasting airline. When traveling from Argentina to Sydney, every flight makes stops in North America, which is very much out of the way. This distance, on a globe, is about the same distance as Argentina to Alaska, but the Sydney flight takes twice as long, and matches up perfectly with the flat earth map. There are many instances where flight paths exactly follow the flat earth map, and look retarded on a globe. I have found at least a half a dozen so far. This is empirical evidence of a flat earth.A little preface for my next proof is some information we weren't indoctrinated on: the actual curvature expected from a ball that is 25,000 miles in circumference. For some reason, we are not taught this in school. Why not? All they can say is it's so ridiculously massive, that's why there's no curve. Every high school student should be able to cite the allowed curvature over any given distance, and be able to demonstrate it in simple experimentation, but we don't, and we can't. The formula for calculating the curvature of the earth is simple. I'm sure you're going to fact check this, because everyone has to, and no one can believe it. For the first mile you should allow 8". Every mile thereafter, you will need to square the mile. So 2 miles should yield 32" of curvature. At three miles, the average human shouldn't be able to see the ground past this point. I'll let Dr Samuel Rowbotham explain it."If the earth is a globe, and is 25,000 English statute miles in circumference, the surface of all standing water must have a certain degree of convexity--every part must be an arc of a circle. From the summit of any such arc there will exist a curvature or declination of 8 inches in the first statute mile. In the second mile the fall will be 32 inches; in the third mile, 72 inches, or 6 feet, as shown in the diagram below.

"Let the distance from T to figure 1 represent 1 mile, and the fall from 1 to A, 8 inches; then the fall from 2 to B will be 32 inches, and from 3 to C, 72 inches. In every mile after the first, the curvature downwards from the point T increases as the square of the distance multiplied by 8 inches. The rule, however, requires to be modified after the first thousand miles."The following table will show at a glance the amount of curvature, in round numbers, in different distances up to 100 miles.Statute Miles AwayMaths= Drop1mile 1 x 1 x 8 =8 Inches2 miles 2 x 2 x 8 =32 Inches3 miles 3 x 3 x 8 / 12 =6 Feet4 miles 4 x 4 x 8 / 12 =10 Feet5 miles 5 x 5 x 8 / 12 =16 Feet6 miles 6 x 6 x 8 / 12 =24 Feet7 miles 7 x 7 x 8 / 12 =32 Feet8 miles 8 x 8 x 8 / 12 =42 Feet9 miles 9 x 9 x 8 / 12 =54 Feet10 miles 10 x 10 x 8 / 12 =66 Feet"To find the curvature in any number of miles not given in the table, simply square the number, multiply that by 8, and divide by 12. The quotient is the curvation required."

As you can see, the curvature of the earth should be very noticeable, for instance, when flying from California to Hawaii, about 2,500 miles, you are flying over a 760 mile high hump of water. This math has been confirmed in cad software. Curvature can be proved wrong with simple magnification techniques like telescopes, binoculars, and the flat earther's favorite tool, the Nikon p900.

2. Distances sightedIn an instance where telephoto lenses are not even necessary, a "mirage" was photographed from Michigan overlooking the Chicago skyline, a span of 60 miles, which should be 2,400 ft below the supposed curvature of the earth. When we look at the heights of the buildings in Chicago, we find that The tallest building at 1,450 ft is the Sears tower. We should not be able to see ANY of Chicago from Michigan. Now before you go agreeing with the whole bs mirage thing, think to yourself what an actual mirage looks like, usually inverted, mirrored, partially opaque, and generally distorted, Google some images of actual mirages. Conditions are perfect indeed. This is not a mirage.There are many instances of long distances being seen, and this is something that anyone can confirm, and is empirical evidence of a flat earth.http://www.abc57.com...

3. Reflection of light on flat waterStating the obvious, the natural physics of water is to find and maintain a level surface, and has never been shown to be able to curl around a spinning ball (this "fact" is introduced young so it doesn't sound as ridiculous as it does now). Take a look at sunsets on water as the sun kisses the water, the reflection extends in a straight line, extending all the way to the viewer, or edge of the water. Warp a piece of sheet metal and hold it up to a bulb, you will never reproduce what is seen in water until you flatten the metal. This is empirical evidence of a flat earth.

Firstly, If you look out the window on an airplane or any other object that goes up at high attitudes , you can see that the earth curves, proving that Earth is a sphere, and not flat. Or if willing to pay up a small fortune, you can see the Earth curve on a recreational spaceship. Not only that photos from the 1960s and 1970s show that the earth is a sphere, not flat. And don't forget the photos and videos of the International Space Station's (ISS). Not only that, you can see more the higher your up. Here is my example; you are on a massive field of grass (Just for the example, this field is 500 Million Kilometers in area), for some reason you can see 0.1 of Kilometer more every 50 meters. The field is 0 meter sea level. You are on the field at 0 meter sea level and you can see for 10 Kilometers. Then, a 600 meter hill comes up, you decide to climb this hill. At the top you can now see for 11.2 Kilometers. This is because of the earth's curve. These pictures will show it works.http://www.smarterthanthat.com...http://www.smarterthanthat.com...
Secondly, if the earth was flat, then how come there is day and night? On a spherical earth, the planet rotates counter-clockwise, making it have day and night cycles for around 12 hours, unlike the flat earth. You see, if there was a flat earth, then the sun will be beaming light and other products of the sun at all places. This will turn the earth extremely hot, killing all life on the entire "flat" earth.
Thirdly, if there was a flat earth, then what happened to the people who traveled to the South Pole? Also, the UN flag doesn't mean it supports the flat earth theory. In fact this is a quote from the UN itself:

A map of the world representing an azimuthal equidistant projection centred on the North Pole, inscribed in a wreath consisting of crossed conventionalized branches of the olive tree, . . . The projection of the map extends to 60 degrees south latitude, and includes five concentric circles.

Official Seal and Emblem of the United Nations, Report of the Secretary-General, 15 October 1946

You see, an azimuthal equidistant projection is map type that was used by the Flat Earth Society. Ancient Egyptians may have actually used it in star maps in holy books. Also, just because it is a map projection doesn't mean it is right or accurate. For example, the Mercator map is inaccurate. They used it because it doesn't show an actual map of the earth, it shows the Earth as whole.

It seems my "lack of intelligence" has caused my opponent to pull out all the stops to try and defend his beloved ball. Most of them, thoroughly refuted in the links I provided. I will further explain my opponents arguments. Firstly, planes' windows are convex, which curves lines. Secondly, even if the Earth was round you could not see the curve from a plane window. Thirdly, if I were willing to pay a small fortune where could I go to see the Earth from space? Virgin has been trying to do it for years with some 800 investors now demanding their money back because it is not possible.http://www.fool.com...
We supposedly went to the moon in the sixties with little more than a Gameboy, and we haven't put any McDonald's on the moon yet? Your cellphone has more power than what we went to the Moon with. Have you not seen a Funny Thing Happened on the Way to the Moon?https://youtu.be...
The pictures were fake. The astronauts never went to the moon they never left low Earth orbit. The ISS is a fraud, this can easily be done with a U-2 spy plane with a fisheye lens on a camera. They claim to be in the thermosphere which reaches temperatures of some 2000 degrees, and you believe that? Google what elements can withstand this heat.https://youtu.be...
Seeing longer distances at higher altitudes has nothing to do with the curvature of the earth. See in the cute little drawing you presented how the object the viewer is looking at is leaning away from him, and is below eye level? Neither of these is happening in reality. Watch a high altitude balloon launch, the horizon is always eye level, and perfectly flat, and things do not lean away from you the higher you go. Seeing farther from higher altitudes is due only to another aspect of linear perspective. As you gain altitude, you lose some view of the ground directly below you (unless you are on a hill) and gain that of longer distances. if you were to put a camera inches from the ground on one end of a football field ( which is flat) and walk away from it, on the other end you would see only about from the waist up, raise the camera, and the person comes back fully into view. This is another one of those "I can't believe he just said that" proofs, yet again, refuted.http://whotfetw.com...
"Secondly, if the earth was flat, then how come there is day and night?"

The sun is close, and small, illuminating locally. It follows the same laws of perspective as everything else. We know this because it shrinks as it goes away from us.https://youtu.be...

"Thirdly, if there was a flat earth, then what happened to the people who traveled to the South Pole? "

There is no south pole, and we'll never get to go there. You've never heard of the Antarctic treaty? Your buddies guard the edge of our earth like it's made of gold.

"Also, the UN flag doesn't mean it supports the flat earth theory. In fact this is a quote from the UN itself:

A map of the world representing an azimuthal equidistant projection centred on the North Pole, inscribed in a wreath consisting of crossed conventionalized branches of the olive tree, . . . The projection of the map extends to 60 degrees south latitude, and includes five concentric circles."

I didn't say that the UN supports the Flat Earth theory, but they know the Earth is flat.

"... just because it is a map projection doesn't mean it is right or accurate."

Take a good look at the list of map projections I provide, and scroll down to the azimuthal equidistant projection. Now scroll out to the far right, and you can see that is is the one and only map used by the USGS. Now I'm sure with your level of intelligence, you know who the USGS is. They use it because it is correct, the only correct map.https://en.m.wikipedia.org...
So, going into round 3 we have 3 unrefuted, empirical proofs of a flat earth, and 0 for a ball. Let's widen the gap a but, since my opponent has not even addressed any of my arguments.

4. Gyroscopes
Gyroscopes stay on a constant axis from which they are started, even when you sprinkle some magic fairy dust on them. If you've ever had a toy gyro, you would know this. This training manual explains it further. https://youtu.be...
With this in mind, think about how much the earth is supposedly moving about in space. Spinning faster than the speed of sound (where's the sonic boom, and why can I hear those sounds both east and west of me equally?) wobbling, flying around the sun, which is flying around the galaxy, which is flying around the universe. None of these preposterous movements at preposterous speeds can be felt or measured at all. My question is, how are gyroscopes reliable? Planes are outfitted with gyros to tell where the horizon is. Why don't they roll back as the plane goes over the hump?

The 'pro' guy tried to use light refraction (snells law) to prove his flat earth stuff right there in round 2 paragraph 6. He posted "This, in combination with atmospheric refraction, which is very strong close to the ocean's surface, where waves constantly break, throwing salty water in the air. blah blah blah blah blah" then turn around and calls me gullible, so im done here.